THERMODYNAMICS Predicting LLE in Mixed-Solvent Electrolyte Systems by an Electrolyte EOS Julian Youxiang Zuo and Dan Zhang DB Robinson Research Ltd., Edmonton, Alberta, Canada T6N 1E5 Walter Furst ¨ Laboratoire Reacteurs et Processus, ENSMP-ENSTA, 75739 Paris, Cedex 15, France ´ This work extends the pre®iously published aqueous electrolyte equation of state ( ) ( ) AEEOS to predict liquid  liquid equilibria LLE of mixed-sol®ent electrolyte sys- tems. Interaction parameters between ions and organic sol®ents, and cations and anions were determined by fitting the experimental ®apor-pressure data of binary methanol q halide electrolyte mixtures, and then correlated to the cationic Stokes and anionic Paul - ing diameters. The focus is on the ionic standard r reference state and the standard Gibbs energy for transferring salts from one sol®ent to another. The methods applied to predict LLE of se®eral ternary water q organic sol®ent q salt system are to select: 1. the hypo- thetical ideal gas at unit mol fraction, the system temperature, and 1 bar as the ionic standard state; 2. the infinite dilution in the sol®ent mixture as the ionic reference state, whose acti®ity coefficients were con®erted to those at the infinite dilution in pure water by the ionic standard Gibbs energy of transfer. The predicted LLE results agree well with the measured data without any adjusted parameters in fitting the ternary experi - mental data. The extended AEEOS is comparable to the model of Z erres and Prausnitz, but the latter requires two adjusted parameters in fitting the ternary experimental data for each ternary system. Introduction In the past few decades, much progress has been made in representing the thermodynamic properties of electrolyte so- lutions. However, most of the studies in this field are re- stricted to aqueous electrolyte systems. Relatively less atten- tion has been given to mixed-solvent electrolyte systems. Fur- thermore, less emphasis has been put on liquid  liquid equi- Ž . Ž . libria LLE than vapor  liquid equilibria VLE in mixed- solvent electrolyte systems. An accurate and consistent repre- sentation of LLE in mixed-solvent electrolyte systems re- mains one of the challenging research fields in chemical engi- Ž . neering, as pointed out by Liu and Watanasiri 1996 . Ž To represent phase equilibria in mixed-solvent or aque- . ous electrolyte systems, the excess Gibbs energy function is Ž . usually expressed as the sum of a Debye-Huckel DH and a ¨ Ž short-range term. The UNIQUAC model Sander et al., 1986; . Macedo et al., 1990; Li et al., 1994a , or the UNIFAC model Correspondence concerning this article should be addressed to J. Y. Zuo. Ž . Kikic et al., 1991; Achard et al., 1994 was employed for the short-range term. As we know, the short-range term is ob- Ž . tained within the Lewis-Randall LR framework, while the Ž . long-range DH term is derived from the McMillan-Mayer Ž . MM framework. To maintain consistency, the activity coef- ficients need to be converted from the MM to the LR frame- work. Ž . Zerres and Prausnitz 1994 developed a thermodynamic framework for calculating VLE and LLE in ternary systems containing water, alcohol, and a salt. Short-range ion-solvent forces are taken into account primarily by a chemical equilib- rium method based on stepwise ion solvation. Nonideality of water-cosolvent is described by an extended equation of the van Laar form. Long-range electrostatic forces among ions are taken into account by an extended DH model with cor- rections for transferring from the MM to the LR framework Ž . using the method of Cardoso and O’Connell 1987 . The method of Zerres and Prausnitz is powerful because it is ca- November 2000 Vol. 46, No. 11 AIChE Journal 2318